1. Field of the Invention
The present invention relates to a tire pressure monitoring system which directly measures a tire pressure.
2. Description of the Related Art
A TPMS (Tire Pressure Monitoring System) monitors pressures of respective tires while a vehicle is traveling, and notifies reduction of a tire pressure to the driver of the vehicle when the pressure of a tire decreases while the vehicle is traveling. Installation of such a TPMS in a vehicle to be newly sold becomes an obligation in North America.
TPMSs can be roughly divided into an indirect type and a direct type (sensor type).
A typical example of the indirect type is an ABS (Anti-lock Braking System) type which uses a wheel speed sensor of ABS provided for each wheel. As the sensor of an existing system can be used, measurement (estimation) of a tire pressure can be realized inexpensively.
Conversely, the direct type (sensor type) directly measures a tire pressure through an (air) pressure sensor while a vehicle is traveling, and transmits a measured value to a receiving unit in a vehicle main body by means of a radio wave, thereby directly measuring the tire pressure. The direct type has a higher reliability than the indirect type.
Because the direct type TPMS transmits a tire pressure to the receiving unit in the vehicle main body by a radio wave, it is necessary for the TPMSs to store the identification numbers (ID) of respective pressure sensors in order to distinguish running tires mounted on a local vehicle from tires mounted on other vehicles.
Accordingly, when, for example, wheels are changed from those of normal tires (summer tires) to those of studless tires (winter tires), it is requisite to register the sensor IDs in a monitoring unit provided in the vehicle main body at a dealer, etc., using a special apparatus. Thereafter, when the wheels are changed back from those of the winter tires to those of the normal tires (summer tires), it is necessary to register the sensor IDs of the wheels of the normal tires (summer tires) again.
According to the TPMS disclosed in Japan Patent No. 3815305, when a centrifugal force sensor provided in a pressure sensor of a spare wheel (spare tire) loaded in a local vehicle or of a wheel loaded for replacement is activated due to hard vibration originating from a rough road and the tire pressure sensor unit transmits data on a tire pressure even though such wheel is not mounted as a running wheel, the monitoring unit may wrongly register the sensor ID of that tire pressure sensor unit as the sensor ID of the running wheel.
Japan Patent No. 3815305 discloses that the sensor ID of a monitoring target is only changed, and changing of a set value (determination threshold) tire by tire for properly determining tire pressure reductions are not taken into consideration. Moreover, replacement of plural wheels (tire set) such as replacement from wheels of summer tires to wheels of winter tires is not taken into consideration of disclosure of Japan Patent No. 3815305.